testing: Switch to Debian stretch base images

Author: tobiasbrunner

testing/testing.conf

@@ -49,7 +49,7 @@ fi
 # Base image settings
 # The base image is a pristine OS installation created using debootstrap.
 : ${BASEIMGSIZE=1600}
-: ${BASEIMGSUITE=jessie}
+: ${BASEIMGSUITE=stretch}
 : ${BASEIMGARCH=amd64}
 : ${BASEIMG=$IMGDIR/debian-$BASEIMGSUITE-$BASEIMGARCH.$IMGEXT}
 : ${BASEIMGMIRROR=http://http.debian.net/debian}